The process of solid-state extrusion is known. Extrusion processes that are used include ram extrusion and hydrostatic extrusion. Ram extrusion utilises a chamber in which polymer billets are placed, one end of which contains a die and the other an axially mobile ram. The billet is placed within the chamber such that the sides of the billet are touching the sides of the chamber. The mobile ram pushes the billets and forces them through the die.
In hydrostatic extrusion processes, the billet is of a smaller size than the chamber and does not come into contact with the sides of the chamber. The chamber contains a pressure generating device at one end and a die at the other. The space between the billet and the chamber is filled with a hydraulic fluid, pumped into the chamber at the end containing the pressure generating device. During operation pressure is increased on the hydraulic fluid and this in turn transmits pressure to the surface of the billet. As the billet passes through the die some of the hydraulic fluid adheres to the surface of the billet, providing additional lubrication to the process.
Both processes produce a polymer that is oriented in a longitudinal direction, having increased mechanical properties, such as tensile strength and stiffness. However, the orientation in a longitudinal direction can also make the polymer weak and subject to transverse cracking or fibrillation under abrasion. The process of pushing the polymer through a die can also create surface imperfections caused by frictional forces.
U.S. Pat. No. 5,204,045 to Courval et al. discloses a process for extruding polymer shapes with smooth, unbroken surfaces. The process includes heating the polymer shape to below the melting point of the polymer and then extruding the polymer through a die that is heated to a temperature at least as high as the temperature of the polymer. The process also involves melting a thin surface layer of the polymer to form a thin, smooth surface layer. The process produces a material of a uniform appearance and subsequent commercial applications are limited as a result.